1. Field of the Invention
The present invention relates to a center differential unit including a planetary gear, and particularly to a center differential unit containing a planetary gear used for effectively transmitting power from an engine to driving wheels of a vehicle, which can produce a sufficient differential toque and has an excellent durability.
2. Discussion of the Related Art
In a four wheel drive, power from an engine is transmitted to front and rear wheels usually by a center differential unit. In other words, power from the engine is first transmitted to a transmission by way of a clutch. The transmission output is divided by a center differential unit and transmitted to a front differential unit and a rear differential unit. Torque decelerated by the front and rear differential units is conveyed respectively to the front and rear wheels.
The center differential unit of a planetary gear type is widely used since it is relatively compact and changes in torque delivery to front and rear wheels is easily controlled.
Japanese Patent 2652715 discloses a four wheel drive with a center differential unit having a planetary gear. The center differential unit will now be explained by referring to a schematic cross section of a center differential unit shown in FIG. 5 and a schematic diagram in FIG. 6.
An output shaft 101 is connected with a rear drive shaft 102, which transmits power to a rear differential unit by way of a propeller shaft. The output shaft 101 and the rear drive shaft 102 are provided to rotate coaxially via bushings 103 and thrust bearings 104. A reduction gear 105 is supported by the output shaft 101 via the bushings 103 and rotates on the output shaft 101. The reduction gear 105 and the output shaft 101 are supported on a gearbox casing 100 by ball bearings 106. A center differential unit 110 is provided over the output shaft 101, the rear drive shaft 102 and the reduction gear 105 coaxially therewith. The reduction gear 105 meshes with a reduction gear 108 provided on a front drive shaft 107. Here, the front drive shaft 107 functions to transmit power to a front differential unit.
The center differential unit 110 includes a large diameter sun gear 111 on the output shaft 101 of a transmission on the input side, a small diameter sun gear 112 on the rear drive shaft 102 on the output side, a small diameter pinion 113 meshed with the large diameter sun gear 111, and a large diameter pinion 114 meshed with the small diameter sun gear 112. The small diameter pinion 113 and the large diameter pinion 114 are provided on a single pinion member 115.
Moreover, a supporting plate 117 for a carrier 116 is attached to the rear drive shaft 102 by way of a ball bearing 118. The supporting plate 117 and the reduction gear 105 are united as an integral body, and a pinion shaft 119 extends over the supporting plate 117 and the reduction gear 105. The pinion member 115 is supported by the pinion shaft 119 having needle bearings 120 therebetween. Both ends of the pinion member 115 are supported on the reduction gear 105 and the supporting plate 117 by a thrust washer 122 interposed therebetween. An oil supply hole 121 is perforated on the pinion shaft for supplying oil to the needle bearings 120. For the pinion shaft 119, high-carbon steel or case-hardened steel which has been subjected to a thermal treatment for hardening the surface.
In the above-mentioned center differential unit 110 containing a planetary gear of a compound type without a ring gear, differential function is attained by using the large diameter sun gear 111 and the small diameter sun gear 112 with an appropriate number of teeth. A disproportional torque ratio on the rear wheels is provided by appropriately distributing the output torque of the transmission. This is achieved by appropriately setting the distance from the meshing point of the large diameter sun gear 111 and the small diameter pinion 113 to the center of the large diameter sun gear 111, and the distance from the meshing point of the small diameter sun gear 112 and the large diameter pinion 114 to the center of the small diameter sun gear 112.
Alternatively, it is possible to use helical gears for the large diameter sun gear 111, small diameter sun gear 112, small diameter pinion 113 and large diameter pinion 114 so as to generate a thrust load. The thrust load causes friction between the edges of the thrust washer 122 and the pinion member 115. In addition, force resulting from a separation load and a tangential load is applied to the small diameter pinion 113, large diameter pinion 114 and a contacting face between the pinion member 115 and the pinion shaft 119 (needle bearings 120). Accordingly, frictional force is generated on the needle bearings 120. Thus, a differential control mechanism is provided.
In Japanese Patent 2652715, a radial load is applied to the pinion member 115 depending upon an input torque. In order to eliminate a seizure of the pinion shaft 119 with the pinion member 115, and the galling of these, the needle bearings 120 are provided as bearings for the pinion member 115. The needle bearings 120 can be replaced by bushings.
The needle bearing 120 is durable against a large load, but has a small frictional resistance, since it is a rolling bearing. On the other hand, the bushings contact with the pinion member 115 by sliding therebetween. Therefore, the bushing has a large frictional resistance, but a seizure will occur when lubricating oil is in sufficiently supplied. Accordingly, it is difficult to take an appropriate countermeasure when large torque for reducing differential action is required based on a load when traveling for instance on a muddy road or a road with a bad surface condition.
Usually, metal materials are employed for the pinion member 115 and the pinion shaft 119 in order to attain a sufficient strength. When large torque for reducing differential action is necessary, it is possible to increase friction between the pinion member 115 and the pinion shaft 119 by supporting the pinion member directly on the pinion shaft 119 without providing the needle bearing 120 or the bushing. In this case, however, it is impossible for the pinion shaft 119 and the pinion member to endure such a large stress, which causes seizure. Alternatively, if forced lubrication is carried out so as to prevent the seizure, the frictional resistance between the pinion shaft 119 and the pinion member 115 is decreased.
In order to obtain sufficient friction, it is effective to use a paper material, which is usually used as a friction material for a multi-plate clutch for an automatic transmission, by interposing the paper material between the pinion shaft 119 and the pinion member 115. However, the paper material is not suitable for practical use when lubricating oil containing an additive of a mineral material is used for the differential gear. This is because the durability of the paper friction material is largely decreased due to absorption of such oil.
In this way, it is difficult to obtain both sufficient friction and torque for reducing differential action.
It is therefore a first object of the present invention to provide a center differential unit of a planetary gear type which provides torque for reducing differential action corresponding to an input torque in addition to maintaining a stable function of a planetary gear contained therein by obtaining a sufficient frictional force between a pinion shaft and a pinion member without interposing a bearing or a bushing therebetween and efficiently eliminating a seizure therebetween, comprising a carrier; a first sun gear for receiving an output from a transmission and positioned interiorly of the carrier; a second sun gear provided on an output side with respect to the first sun gear and positioned interiorly of the carrier; a first pinion meshing with the first sun gear; a second pinion meshing with the second sun gear; a pinion member carrying the first pinion and the second pinion therearound, the pinion member having a shaft-receiving hole; and a pinion shaft having at least a surface of carbon/carbon composite, the pinion shaft supported by the carrier at one end on the output side and directly supporting the pinion member so as to rotate by being inserted into the shaft-receiving hole of the pinion member.
The first object of the present invention can also be achieved by the center differential unit of the planetary gear type, wherein the pinion shaft is made entirely of carbon/carbon composite.
A second object of the present invention is to provide the center differential unit which provides torque for reducing differential action with an improved reliability with efficiently maintaining a sufficient frictional force between the pinion shaft and the pinion member contained in the planetary gear, wherein force resulting from a separation load and a tangential load generated at a meshing point of the first sun gear and the first pinion and a meshing point of the second sun gear and the second pinion is applied to the pinion shaft.
A third object of the present invention is to provide the center differential unit which provides a larger quantity of torque for reducing the differential action, wherein the first sun gear, the second sun gear, the first pinion and the second pinion are helical gears, a thrust load generated at a meshing point of the first sun gear and the first pinion, and a thrust load generated at a meshing point of the second sun gear and the second pinion cause frictional force between the second sun gear and the carrier.
A fourth object of the present invention is to provide a center differential unit of a planetary type which provides torque for reducing differential action corresponding to an input torque with maintaining a stable function of a planetary gear contained therein and a sufficient frictional force between a pinion shaft and a pinion member without further providing a bearing or a bushing as a separate member therebetween and by efficiently eliminating a seizure therebetween, comprising a carrier; a first sun gear for receiving a output from a transmission and positioned interiorly of the carrier; a second sun gear provided on an output side with respect to the first sun gear and positioned interiorly of the carrier; a first pinion meshing with the first sun gear; a second pinion meshing with the second sun gear; a pinion member carrying the first pinion and the second pinion therearound, the pinion member having a shaft-receiving hole; a bearing made of carbon/carbon composite provided in the shaft-receiving hole; and a pinion shaft made of a steel material, the pinion shaft supported by the carrier at one end on the output side and directly supporting the pinion member so as to rotate by being inserted into the shaft-receiving hole of the pinion member.
A fifth object of the present invention is to provide the center differential unit which provides torque for reducing differential action with an improved reliability with efficiently maintaining a sufficient frictional force between the pinion shaft and the pinion member contained in the planetary gear, wherein force resulting from a separation load and a tangential load generated at a meshing point of the first sun gear and the first pinion and a meshing point of the second sun gear and the second pinion is applied to the pinion shaft.
A sixth object of the present invention is to provide the center differential unit which provides larger quantity of torque for reducing differential action wherein the first sun gear, the second sun gear, the first pinion and the second pinion are helical gears, a thrust load generated at a meshing point of the first sun gear and the first pinion, and a thrust load generated at a meshing point of the second sun gear and the second pinion cause frictional force between the second sun gear and the carrier.
A seventh object of the present invention is to provide the center differential unit containing a planetary gear having a smaller member, wherein the carbon/carbon composite has a bending strength in the range of 200 MPa to 1200 MPa, a compressive strength in the range of 100 MPa to 800 MPa, and an interlaminar shear strength in the range of 20 MPa to 600 MPa.
A eighth object of the present invention is to provide a planetary gear which functions in a stable manner and is light in weight and simple in structure by having a sufficient frictional force between a pinion shaft and a pinion member without providing a bearing or a bushing therebetween and efficiently eliminating a seizure therebetween, comprising a carrier; a sun gear provided interiorly of the carrier; a pinion meshing with the sun gear; a pinion member carrying the pinion therearound, the pinion member having a shaft-receiving hole; and a pinion shaft having at least a surface of carbon/carbon composite, the pinion shaft supported by the carrier and directly supporting the pinion member so as to rotate by being inserted into the shaft-receiving hole of the pinion member.
The eighth object of the present invention can also be achieved by the planetary gear wherein the pinion shaft is made entirely of carbon/carbon composite.
An ninth object of the present invention is to provides a planetary gear which is light in weight and simple in structure by maintaining a sufficient frictional force between a pinion shaft and a pinion member without further providing a bearing or a bushing as a separate member therebetween and efficiently eliminating a seizure therebetween, comprising a carrier; a sun gear provided interiorly of the carrier; a pinion meshing with the sun gear; a pinion member carrying the pinion therearound, the pinion member having a shaft-receiving hole; a bearing made of carbon/carbon composite provided in the shaft-receiving hole; and a pinion shaft made of a steel material, the pinion shaft supported by the carrier and directly supporting the pinion member so as to rotate by being inserted into the shaft-receiving hole of the pinion member.
A tenth object of the present invention is to provide the planetary gear having a smaller member, wherein the carbon/carbon composite has a bending strength in the range of 200 MPa to 1200 MPa, a compressive strength in the range of 100 MPa to 800 MPa, and an interlaminar shear strength in the range of 20 MPa to 600 MPa.